[0001] The present invention relates to a flowing-water driveable turbine assembly to be
located in river or sea areas with unidirectional, or bi-directional, water flows
to convert the kinetic energy of the water flow into a more easily transferable form
of energy, like, for example, hydraulic energy or electrical energy.
[0002] It is well known to drive a turbine by flow of water. The extraction of kinetic energy
from the water flow causes a reduction in the momentum of the passing water which
in turn creates large reaction forces on the turbine. These reaction forces manifest
themselves primarily as a drag force acting in the direction of water flow. For example,
turbines can weigh anywhere from 800kg up to 200 tonnes and can have a rotor diameter
between 4 and 21m. Smaller turbines can have a thrust of around 45kN while the larger
turbines may have a thrust of around 1MN (equivalent to 100 tonnes) under typical
water flow of around 6 knots. Thus, a flowing-water driveable turbine assembly must
be firmly anchored. Often turbine assemblies are anchored in deep water to avoid interaction
with waves found near the surface or to avoid collisions with boats. However, turbines
must be transportable to an anchoring site. Turbine assemblies must also be accessible
for maintenance and repair in a reasonable amount of time. These aspects of water-driveable
turbine technology pose substantial engineering challenges. Fluid dynamics can be
complex to simulate effectively and moreover real world flow conditions can vary significantly.
As a result many paper proposals for turbines are found not to be practicable. A simple
but effective mounting configuration would ameliorate the engineering challenges and
make practical use of tidal or current energy viable, solving a long desired and environmentally
beneficial challenge.
[0003] Patent publication No.
WO99/02853 discloses a stream turbine to cover large areas of water streams and which can be
manufactured in a ship yard and transported to a site of use and be anchored there.
The stream turbine as a whole can be floated and towed for maintenance or repair.
[0004] Patent publication Nos.
WO2004/085845 and
WO2005/061887 disclose support structures for supporting water current turbines in a sea or river
estuary. The support structure with turbines may be floated to the water surface for
maintenance or repair.
[0005] Patent publication
US 6091161 describes a method of controlling a tethered, underwater, water current-driven turbine,
power-generating device. A predetermined maximum depth and a predetermined minimum
depth are set. In response to sensing depth of the device, an ascend protocol or a
descend protocol are selectively invoked. These protocols maintain an operating depth
of the device that is midway between the predetermined maximum depth and the predetermined
minimum depth. The turbine includes variable-pitch rotor blades. A maximum allowable
drag force load on the turbine rotors is selected. The pitch of the variable-pitch
rotor blades on the turbine is adjusted such that the drag force loading of the device
does not exceed a maximum design level.
[0006] Patent publication
GB 2460309 describes an apparatus comprising a turbine to be driven when submerged in a current
of water; a lifting body for generating a lift force upon the turbine when the turbine
is submerged in the current; the apparatus is secured to a tether; and a stabilizer
for dynamically stabilizing the apparatus in a first configuration in the current
when the current flows in a first direction, and in a second configuration when the
current flows in a second different direction. The geometrical relationship between
the securing points, the centre of drag on the apparatus and the centre of lift of
the apparatus in the first configuration is different to the corresponding geometrical
relationship in the second configuration.
[0007] WO 2009/004420 A2 describes a method of controlling a tethered, underwater, water current-driven turbine,
power-generating device. The device is comprised of dual turbines and dual rotor blades
turned by fluid flow, each turbine including one or more generators driven by rotor
blades. The device is connected by device tethers to a strut, which is moveable to
control depth of the device. The strut is connected to the ocean floor by tripodal
tethers: a main tether, a left side tether and a right side tether, which are strut
control tethers. One or more winches are controlled to maintain operation of the device
within set parameters by varying the tension on one or more of the strut control tethers.
[0008] The present invention is set out in the appended independent claim, with preferred
features set out in the dependent claim.
[0009] Described herein is a turbine assembly with variable net positive buoyancy for generating
power from water currents. The turbine assembly can be slightly positively buoyant
to enable, for example, installation and retrieval. The turbine assembly can also
be anchored, for example to a sea bed or river bed. When anchored, the variable net
buoyancy of the turbine assembly can be increased to enable the turbine assembly to
be positioned at different depths in a body of water. When positioned in this way,
in a substantially fixed position, the turbine assembly can generate power from water
currents at selected depths in a body of water. The variable net buoyancy is provided
by a combination of fixed and variable buoyancy in the turbine assembly. The anchoring
system provides an opposing force to the force provided by the net positive buoyancy
to hold the turbine assembly at the selected depth.
[0010] In an aspect of the present invention there is provided a submersible turbine assembly
comprising a frame sized and configured for supporting one or more flowing water driven
turbines at a predetermined depth range below the surface of a water body having a
bed, the frame including at least one fixed buoyancy component and at least one variable
buoyancy component and optionally one or more hydrodynamic lift-generating surfaces,
the one or more flowing water driven turbines being expected to produce a drag force
which varies with water velocity, the assembly being arranged to be maintained in
a predetermined position above the water body bed and submerged below the expected
water body surface by at least two upstream taut mooring line runs and at least one
downstream taut mooring line run, wherein the mooring line runs are arranged to be
anchored to the water body bed at respective anchor points spaced apart from a point
below the expected position of the assembly by at least the height of the assembly
above the water bed, the frame having attachment points for said taut mooring line
runs comprising upper and lower attachment points for spaced apart attachment of upper
and lower or primary and secondary mooring lines or cables forming each mooring line
run, the attachment points arranged to permit movement of at least some of the mooring
lines relative to the frame during installation and having an arrangement for locking
the mooring lines relative to the frame during use, the assembly being arranged so
that in use in a water current having a nominal flow velocity of 3 metres per second
both the upstream and downstream mooring line runs remain taut and wherein the net
upward force resulting from the fixed and variable buoyancy and any hydrodynamic lift
minus the weight of the assembly is at least 25% of the drag on the structure and
thrust produced by the at least one turbine.
[0011] We have found through extensive tank-testing and simulation that this particular
configuration provides a simple but effective arrangement that gives workable stability
in a vertical direction, horizontal position keeping and resistance to "toppling"
throughout a wide variety of flow conditions that are likely to be encountered in
most real world applications.
[0012] Preferably the net upward force in a nominal flow velocity of 5 metres per second
is no greater than about 200% of the drag on the structure and thrust. This constraint
retains stability within a practical structure.
[0013] Provided herein is a turbine assembly comprising at least one flowing- water driveable
turbine for generating power from water flow, wherein the turbine assembly has a variable
positive buoyancy in water and the turbine assembly is arranged to be anchored by
an anchoring system to anchoring points on a water bed, wherein the turbine assembly
comprises a buoyancy device comprising a fixed buoyant material, and a ballast tank
configured to be filled with water to reduce the positive buoyancy provided by the
buoyancy device, wherein the fixed buoyant material is arranged around the ballast
tank to provide a cradle of fixed buoyant material around the ballast tank.
[0014] The buoyancy device may be elongate, and arranged to lie with its length along the
direction of predominant water flow, wherein the cradle of fixed buoyant material
is arranged so that the buoyancy provided by the fixed buoyant material is symmetrical
along at least one of the length and width of the ballast tank. The cradle of fixed
buoyant material may be arranged at least partially beneath the ballast tank, and/or
arranged partially above the ballast tank so as to surround the ballast tank. As an
alternative, the cradle of fixed buoyant material may comprise a frame to fix the
positions of one or more turbines relative to one or more buoyancy devices. As a further
alternative, the frame provides the cradle underneath the one or more buoyancy devices.
[0015] The net positive buoyancy may be selected to be sufficient to constrain the turbine
assembly against a reactive downward force provided by an anchoring system, to maintain
that position under external downward vertical forces up to a selected threshold force,
and to enable the turbine assembly to be downwardly displaced in the event that the
downward vertical force exceeds the selected threshold force. The selected threshold
force can be selected to restrain movement of the turbine assembly such that excessive
load is not applied to the mooring lines of the anchoring system. The selected threshold
force may be selected to correspond to the force exerted by a wave having a horizontal
or vertical wave particle velocity of, for example 1 metre per second. The net positive
buoyancy is selected to maintain tension in all of the mooring lines of the anchoring
system.
[0016] The net positive buoyancy of the turbine assembly when the ballast tank is flooded
may be less than 15% of the positive buoyancy when the ballast tank is empty of water,
and the fixed buoyant material may be sufficient to maintain positive buoyancy of
the turbine assembly when the ballast tank is flooded. In another possible arrangement,
the fixed buoyant material and the ballast tank are arranged so that the positive
buoyancy of the turbine assembly when the ballast tank is flooded is less than 5 tonnes.
In one arrangement, the net positive buoyancy of the turbine assembly when the ballast
tank is flooded may be around 200kg. The fixed buoyant material and the ballast tank
may be arranged so that the positive buoyancy of the turbine assembly when the ballast
tank is empty is sufficient to react both the combined hydrodynamic drag and thrust
forces generated by the turbines such that the device does not move down in the water
column under operating conditions when the turbine is operating.
[0017] The turbine assembly may comprise a winch and a tag line connectable to the anchoring
system by a mooring line wherein the winch is operable to pull the mooring line into
the turbine assembly using the tag line, and the breaking stress of the mooring line
is greater than the breaking stress of the tag line. Alternatively, the mooring line
and the tag line are the same type and/or size of line and have substantially equal
breaking stresses. The tag line may be coupled to the mooring line by a coupling member
arranged to engage with a locking means of the turbine assembly, wherein the locking
means is operable to lock the coupling member to fix the mooring line to the turbine
assembly to relieve tensile load from the winch. As an alternative, in a smaller turbine
assembly or where loads are not excessive, the lines can be locked off using the winch.
The coupling member may be tapered to guide the coupling member into the locking means
as the winch winds in the tag line.
[0018] The turbine assembly may comprise a hydrofoil for converting water current into hydrodynamic
lift to provide an upward force on the turbine assembly. For example, the turbine
assembly may comprise a frame comprising a plurality of frame members arranged to
support the turbine relative to the buoyancy device, wherein at least one of the frame
members comprises the hydrofoil. The use of a hydrofoil to provide an upward force
can be used as at least a partial substitute for providing upward force through buoyancy,
thus reducing the net buoyancy required in the assembly.
[0019] Described herein is a turbine assembly comprising at least one flowing- water driveable
turbine for generating power from water flow, wherein the turbine assembly has a buoyancy
device operable to provide variable positive buoyancy in water and the turbine assembly
is arranged to be anchored by an anchoring system to anchoring points on a water bed,
wherein the turbine assembly comprises a frame for supporting the turbine, wherein
the frame comprises a plurality of frame members arranged so that, when in use the
frame supports a turbine in water, a first plurality of the frame members are under
tensile stress, and a second plurality of the frame members are under a compressive
load, wherein the first plurality of frame members are flexible and the second plurality
of frame members are rigid.
[0020] The rigid frame members may be hollow and at least one of them (e.g. a transverse,
horizontal, frame member) may have a cross section selected to provide hydrodynamic
lift. The first plurality of frame members may comprise cables or chains which may
be surrounded by a sheath to reduce the hydrodynamic drag caused by the water flow.
[0021] Described herein is turbine assembly comprising at least one flowing- water driveable
turbine for generating power from water flow, wherein the turbine assembly has a variable
positive buoyancy in water and the turbine assembly is arranged to be anchored by
an anchoring system to anchoring points on a water bed, wherein the turbine assembly
comprises: a plurality of buoyancy devices comprising one or both of a fixed buoyant
material and a ballast tank configured to be filled with water; and a frame coupled
to the plurality of buoyancy devices to support the at least one turbine. One of the
frame and one of the buoyancy devices may comprise a plurality of line couplings for
connecting the frame to an anchoring system by a line, and one of the buoyancy devices
may comprise a winch operable to winch a line through at least one line coupling to
apply tensile force between the turbine assembly and the anchoring system. The turbine
assembly may comprise two flowing-water driveable turbines. At least one of the line
couplings may be arranged beneath the centre of buoyancy of one of the buoyancy devices,
and at least one of the plurality of line couplings is arranged toward the nose or
tail end of said one of the buoyancy devices.
[0022] The turbine assembly is typically arranged to float submerged at a predetermined
depth below the average significant wave height experienced in a body of water. This
predetermined depth may be chosen to avoid excessive loads on the turbine assembly
being caused by the waves.
[0023] Described herein is a turbine assembly comprising at least one flowing- water driveable
turbine for generating power from water flow in a body of water, wherein the turbine
assembly has a variable positive buoyancy in water and the turbine assembly is arranged
to be anchored by an anchoring system to anchoring points on the bed of the body of
water to provide a downward force to constrain the turbine assembly submerged at a
depth below the average significant wave height experienced in the body of water,
wherein, the turbine assembly comprises at least four mutually spaced line couplings
for attaching the turbine assembly to the anchoring system, and the line couplings
are connected to the anchoring system by straight taut lines. One of the turbine assembly
and the lines may comprise positive buoyancy selected so that the maximum deviation
of the lines from a straight line is no more than 5% of the length of the line between
the anchoring system and the turbine assembly, for example the lines may comprise
buoyancy aiding devices.
[0024] Described herein is a method of submerging a turbine assembly having variable positive
buoyancy in water, wherein the turbine assembly has at least one flowing water drivable
turbine for generating power from water flow and at least one winch having a respective
pull line connectable to an anchoring system, wherein the method comprises the steps
of: a) floating the turbine assembly to an installation site; b) coupling the or each
pull line to an anchoring system submerged in water; c) reducing the positive buoyancy
of the turbine assembly; d) operating the or each winch to submerge the turbine assembly
to a target location by force of tension in the or each pull line; e) locking the
winch upon arrival at the target site; and f) increasing the positive buoyancy of
the turbine assembly. Optionally, step e) comprises locking the or each pull line
to the turbine assembly upon arrival at the target site to relieve strain from the
winch.
[0025] The pull line may comprise a tag line and a mooring line having a greater breaking
stress than the tag line, and in which locking the pull line to the turbine assembly
comprises locking the mooring line to the turbine assembly.
[0026] Described herein is an anchoring system for anchoring a positively buoyant turbine
assembly in water, wherein the turbine assembly has at least one flowing-water driveable
turbine for generating power from water flow, wherein the anchoring system comprises
at least three anchoring cables anchorable to at least three mutually spaced anchoring
points on a water bed covering a footprint greater in width and in length than the
turbine assembly, wherein each anchoring point on the water bed is attachable to two
mutually spaced attachment points on the turbine assembly and wherein the anchoring
system is arranged to provide a downward force to constrain the turbine assembly against
the upward force of the positive buoyancy of the turbine assembly.
[0027] The anchoring system may have at least a tripod formation of three anchoring cables
which provides sufficient downward reactive force and stability to constrain a positively
buoyant turbine assembly against the upward force of the of the turbine assembly and
against drag forces caused by water current flowing past the turbine assembly. Advantageously,
inherent flexibility in the anchoring cables can absorb impacts against the turbine
assembly or its rotors. Additional stability can be provided by additional anchoring
cables without diminishing the anchoring system's ability to absorb shocks.
[0028] The top third of the water column (i.e. the optimal position for power extraction)
in a deep water site (i.e. 40m depth or more) is inaccessible to a traditional anchoring
system, such as gravity anchors or columns driven into sea bed, due to the increased
hydrodynamic drag of the structure and the overturning moment caused by water current
drag forces. The top third of the water column may be accessible to the anchoring
system, even at deep water sites, by simply varying the length of the anchor cables.
Advantageously, inherent flexibility in the anchoring cables permits the anchoring
points to be positioned at suitable sites over a large area of the sea bed for example
to accommodate undulations and avoid hazards. This may widen and lengthen the anchoring
system's footprint and, in doing so, increase stability to counteract any increased
overturning moment experienced by the turbine assembly at a greater elevation above
the water bed.
[0029] The attachment points on the turbine assembly may be mutually spaced in at least
a direction of upward force of buoyancy. This provides stiffer resistance to heave
under water flow.
[0030] Each anchoring cable may couple the two attachment points on the turbine assembly
to a single anchor point on the water bed.
[0031] Each anchoring cable, or mooring line run, may bifurcate into a pair of cable branches
for coupling to the pair of mutually spaced attachment points on the turbine assembly.
This provides a constraint on the turbine assembly in all six degrees of freedom.
[0032] Each anchoring cable may comprise a mooring line to constrain the turbine assembly
against the upward force of the positive buoyancy of the turbine assembly, wherein
each anchoring cable comprises a tag line to provide directional support to the turbine
assembly and wherein the tag line branches from the mooring line. Use of different
lines to perform specific functions improves the performance of the anchoring system.
[0033] The length of the mooring line and/or the length of the tag line may be variable.
This permits adjustment of the orientation of the turbine assembly when anchored to
the water bed by the anchoring system.
[0034] The tag line may have greater elasticity than the mooring line. The tag lines absorb
shock and may permit localised movement of the turbine assembly while the mooring
lines maintain the fundamental stiffness of the anchoring system.
[0035] The tag line and the mooring line may be made of different materials. The mooring
line may be made of a high performance material with extra strength such as DYNEEMA
(trade mark) polyethylene rope. The tag lines may be made of a basic material such
as braided nylon thereby economising on cost.
[0036] The tag line may branch from the mooring line at an intermediate point along the
length of the mooring line. This provides a constraint on the turbine assembly in
all six degrees of freedom.
[0037] The at least three anchoring cables may be four anchoring cables diverging outwardly
from the turbine assembly to the water bed in a substantially pyramidal form on the
water bed. This provides increased resistance to heave under water flow and constrains
the assembly in all six degrees of freedom.
[0038] The at least three anchoring cables can also be at least six pairs of anchoring cables,
wherein ends of each pair of anchoring cables are coupled to a respective anchor point
on the water bed, wherein the anchoring cables of each pair of anchoring cables diverge
from said anchoring point to where opposite ends of the anchoring cables are fixed
to a pair of mutually spaced points of the turbine assembly, and wherein at least
three pairs of anchoring cables are fixed to each end of the elongate turbine assembly.
This provides greater stability in water flow.
[0039] An angle of inclination of the anchoring cables from the water bed and with respect
to the horizontal may be no more than about 60 degrees. Alternatively, an angle of
inclination of the anchoring cables from the water bed and with respect to the horizontal
may be no more than about 45 degrees. This is to provide an anchoring system with
vertical stability and without tending to pull the anchoring points out of their holes
in the water bed.
[0040] An angle of inclination of the anchoring cables from the water bed and with respect
to the horizontal may be no less than about 10 degrees. Alternatively, an angle of
inclination of the anchoring cables from the water bed and with respect to the horizontal
may no less than about 15 degrees. This is to provide an anchoring with horizontal
stability and which also provides clearance under the turbine support for water flow.
This can reduce heave on the turbine support.
[0041] An angle of inclination of the anchoring cables from the water bed and with respect
to the horizontal may 30 degrees +/- 15 degrees.
[0042] At least one anchoring cable may have integral resistance to shock and/or at least
one anchoring cable may be connected in series or in parallel with a damper.
[0043] The anchoring cables may be streamlined and/or equipped with vortex suppressants.
This may reduce, or even eliminate, vortex induced vibration caused by water flow
around the anchoring cables. It is also to reduce the hydrodynamic drag of the anchoring
cables.
[0044] Each anchoring cable may be anchored in a respective hole in the water bed.
[0045] Described herein is a submersible turbine assembly comprising at least one flowing-water
driveable turbine for generating power from water flow, wherein the turbine assembly
is positively buoyant in water, wherein the turbine assembly comprises an anchoring
system arranged to provide a downward force to constrain the turbine assembly against
the upward force of the positive buoyancy of the turbine assembly, wherein the anchoring
system comprises at least three anchoring cables anchorable to at least three mutually
spaced points on a water bed covering a footprint greater in width and in length than
the turbine assembly. A turbine assembly embodying this has an anchoring system with
at least a tripod formation of three anchoring cables which provides sufficient stability
to constrain a positively buoyant turbine assembly against the upward force of the
of the turbine assembly and against drag forces caused by water current flowing past
the turbine assembly. Advantageously, inherent flexibility in the anchoring cables
can absorb impacts against the turbine assembly or its rotors. Additional stability
can be provided by additional anchoring cables without diminishing the anchoring system's
ability to absorb shocks.
[0046] Each anchoring point on the water bed may be attachable to two mutually spaced attachment
points on the turbine assembly.
[0047] Each of the two mutually spaced attachment points on the turbine assembly may be
mutually spaced in at least a direction of upward force of the positive buoyancy.
[0048] Each anchoring cable may couple each of the two mutually spaced attachment points
on the turbine assembly to a single anchoring point on the water bed, wherein each
anchoring cable comprises a mooring line to constrain the turbine assembly against
the upward force of the positive buoyancy of the turbine assembly, wherein each anchoring
cable comprises a tag line to provide directional support to the turbine assembly
and wherein the tag line branches from the mooring line.
[0049] Also described herein is an anchoring cable for an anchoring system of a positively
buoyant turbine assembly in water, wherein the anchoring cable comprises a helical
protrusion arranged about the circumference of the anchoring cable to provide a vortex
suppressant. Vortex induced vibration may become apparent after an anchoring system
is installed in water with water currents. Advantageously, the anchoring cable may
avoid the need to retrofit a vortex suppressant system or device in order to reduce
vortex induced vibration, wandering and hydrodynamic drag caused by water flow around
the anchoring cable.
[0050] The anchoring cable may be woven and the helical protrusion may be woven into the
anchoring cable. This integrates production of the helical protrusion into the weaving
process used to make the cable.
[0051] Alternatively, the helical protrusion is bonded to the anchoring cable. This integrates
production of the helical protrusion into the cable manufacturing process.
[0052] The helical protrusion and the anchoring cable may be made of the same material.
This economises on the variety of materials employed.
[0053] The pitch of the helical protrusion may be no more than sixteen times the diameter
of the anchoring cable. Further, the pitch of the helical protrusion may be no more
than twelve times the diameter of the anchoring cable.
[0054] The pitch of the helical protrusion may be no less than four times the diameter of
the anchoring cable. Further, the pitch of the helical protrusion may be no less than
eight times the diameter of the anchoring cable.
[0055] The pitch of the helical protrusion may be between eight and twelve times the diameter
of the anchoring cable. This may optimise the vortex suppressing properties of the
helical protrusion.
[0056] The outer diameter of the helical protrusion may be no more than 200% of the diameter
of the anchoring cable. Further, the outer diameter of the helical protrusion may
be no less than 110% of the diameter of the anchoring cable.
[0057] The outer diameter of the helical protrusion may be between 135% and 175% of the
diameter of the anchoring cable. This may optimise the vortex suppressing properties
of the helical protrusion.
[0058] The anchoring cable may be made of nylon, polypropylene and/or polyethylene material.
These materials are suitable for cables or ropes used under tension and they do not
corrode.
[0059] Also described herein is a turbine assembly comprising at least one flowing-water
driveable turbine for generating power from water flow, wherein the turbine assembly
has a variable positive buoyancy in water and the turbine assembly is arranged to
be anchored by an anchoring system to anchoring points on a water bed, wherein the
positive buoyancy of the turbine assembly is variable between a first upward force
and a second upward force greater than the first upward force, and wherein the first
upward force is sufficient to constrain the turbine assembly against a downward force
of an anchoring system. The positive buoyancy can be reduced when the turbine assembly
is to be submerged. Then, the positive buoyancy can be increased upon arrival at the
target location where a greater uplift force is required to stiffen the anchoring
system. This helps to reduce effort needed to submerge the turbine assembly to its
target location. Advantageously, a smaller winch with a smaller pull line may be used
to perform the submerging process. Alternatively, a remote operated vehicle may be
used instead which can perform the submerging process more quickly against less positive
buoyancy. Variable positive buoyancy presents the operator with advantageous flexibility.
[0060] The turbine assembly may further comprise a turbine support arranged to be anchored
by said anchoring system to a water bed and wherein the at least one turbine is secured
to the turbine support. The turbine support provides capacity for storage of buoyancy
devices.
[0061] The turbine support may have a variable positive buoyancy in water variable between
the first positive buoyancy and the second buoyancy. This avoids the need to attach
buoyancy to the at least one turbine.
[0062] The turbine assembly may further comprise a ballast tank fillable with a positive
buoyancy medium. The ballast tank may be filled with air or another positive buoyancy
medium from a surface vessel, a remote operated vehicle or from compressed air stored
on board the turbine assembly to provide a readily controllable means of adjusting
positive buoyancy.
[0063] The turbine assembly may further comprise means for converting water current into
hydrodynamic lift to provide an upward force on the turbine assembly. This may provide
a passive upward force in addition, or as an alternative, to any active upward force
provided by devices such as a ballast tank fillable with air.
[0064] The means for converting water current into hydrodynamic lift may comprise at least
one hydrofoil. The, or each, hydrofoil may be attached to the turbine assembly during
manufacture.
[0065] The hydrodynamic lift may be variable. This may increase positive buoyancy in proportion
to an increase in surrounding water current speed to progressively counteract any
drag moment created by longitudinal drag.
[0066] The, or each, turbine may comprise a turbine module having a duct for directing water
through the turbine. A duct may shield the turbine from turbulence caused by any adjacent
turbines or wave action and to increase efficiency of energy extraction from the water
current
[0067] As described herein, hydrodynamic lift and/or variable buoyancy in combination with
the fixed buoyancy of the turbine assembly is sufficient to counteract the downward
component of drag caused by water current flowing past the turbine assembly acting
around the anchor points of the anchoring system so that the turbine assembly resists
movement with change in water flow.
[0068] As described herein, the hydrodynamic lift and/or variable buoyancy of the turbine
assembly acting against the downward force of the anchoring system is sufficient to
counteract excursion of the turbine assembly from a target position of more than double
the length of the turbine assembly with a water current at maximum target speed and/or
maximum target wave height. In a further optional arrangement, the hydrodynamic lift
and/or variable buoyancy of the turbine assembly acting against the downward force
of the anchoring system is sufficient to counteract excursion of the turbine assembly
from a target position of more than the length of the turbine assembly with a water
current at maximum target speed and/or maximum target wave height.
[0069] As described herein, the hydrodynamic lift and/or additional buoyancy of the turbine
assembly acting against the downward force of the anchoring system is sufficient to
counteract excursion of the turbine assembly from a target position of more than double
the height of the turbine assembly with a water current at maximum target speed and/or
maximum target wave height. In a further optional arrangement, the hydrodynamic lift
and/or additional buoyancy of the turbine assembly acting against the downward force
of the anchoring system is sufficient to counteract excursion of the turbine assembly
from a target position of more than the height of the turbine assembly with a water
current at maximum target speed and/or maximum target wave height.
[0070] As described herein, uplift provided by the hydrodynamic lift and/or variable buoyancy
of the turbine assembly is no more than 400% of the uplift provided by the fixed buoyancy
of the turbine assembly.
[0071] As described herein, uplift provided by the hydrodynamic lift and/or additional buoyancy
is no less that 80% of the uplift provided by the fixed buoyancy of the turbine assembly.
[0072] As described herein, maximum net upward force of positive buoyancy is no more than
150% of the weight of the turbine assembly in air. In a further optional arrangement,
maximum net upward force of positive buoyancy is no more than 100% of the weight of
the turbine assembly in air. In a further optional arrangement, maximum net upward
force of positive buoyancy is no more than 50% of the weight of the turbine assembly
in air. By minimising the net upward force of positive buoyancy smaller diameter anchoring
cables may be used. This may reduce the weight of the anchoring system. Also, tension
in the anchoring cables may be reduced thereby reducing forces acting upon the anchoring
points.
[0073] As described herein, the first upward force is provided by a fixed buoyant material
attached to the turbine assembly. This may ensure that the turbine assembly always
has some degree of positive buoyancy. It may be towed on the water surface to an installation
site without external intervention. A degree of positive buoyancy aids control while
the turbine assembly is being submerged.
[0074] Also described herein is a turbine assembly comprising at least one flowing water
driveable turbine for generating power from water flow, wherein the turbine assembly
has a positive buoyancy in water, wherein the turbine assembly is arranged to be anchored
by an anchoring system to a water bed, wherein the turbine assembly comprises at least
one winch each with a respective pull line connectable to the anchoring system, and
wherein the or each winch is operable to pull the turbine assembly towards the water
bed by tensile forces acting through the pull line and wherein the or each winch is
lockable against tensile forces acting through the pull line, wherein the positive
buoyancy of the turbine assembly in water has an upward force to constrain the turbine
assembly against a downward force of an anchoring system. Conditions at sea, in rivers
and in estuaries can vary significantly and quickly. Water currents can change direction.
These environmental factors present challenges when installing a large object such
as a turbine assembly under water. Advantageously, the winch may provide the turbine
assembly with means of self- propulsion which may submerge it directly to its installation
site, despite unfavourable environmental conditions of the surrounding water, where
it can be locked to the turbine assembly with a docking connector (locking means)
for the mooring line, such that the tensile load in the mooring line is not borne
by the winch once the mooring line is connected. This also avoids the need for a large
surface vessel with a heavy lifting crane to submerge the turbine assembly to its
installation site or subsequently retrieve it. This may eliminate problems associated
with the motion of the surface vessel pulling in an unpredictable manner on the turbine
assembly.
[0075] As described herein, the positive buoyancy of the turbine assembly is variable. The
positive buoyancy can be reduced when the turbine assembly is being submerged. Then,
the positive buoyancy can be increased when the target location is reached and a greater
uplift force is required. This helps to reduce tension in the pull lines and minimise
effort required on the part of the winch, or winches, during submerging process. Smaller
diameter anchoring cables may be used if these also perform the role of pull lines.
[0076] As described herein, the or each winch is operable externally. This economises on
the weight and expense of having a motor permanently coupled to the winch.
[0077] The or each winch may be operable by an electric motor coupled thereto and wherein
the electric motor is controllable remotely. This makes the turbine assembly fully
self-propelled. The or each winch may be hydraulic, and the turbine assembly may comprise
hydraulic power unit arranged to provide power to at least one winch.
[0078] As described herein, the or each winch is operable by a remote operated vehicle coupled
thereto.
[0079] As described herein, the turbine assembly has variable positive buoyancy. The turbine
assembly may have increased positive buoyancy during towing and in use and the turbine
assembly may have reduced positive buoyancy while being submerged to its installation
site thereby reducing the load on the or each winch.
[0080] As described herein, the at least one winch comprises at least three winches each
with a pull line for connection to a part of a respective anchoring cable of the above
mentioned anchoring system having at least three anchoring cables. This may provide
the turbine assembly with means of self-propulsion directly to its optimal position
at the installation site. Advantageously, the three winches may be used to vary the
position (i.e. pitch or roll) of the turbine assembly.
[0081] As described herein, each pull line is integrally connected to a part of a respective
anchoring cable. This anchoring cables double-up as pull lines thereby making economic
use of material.
[0082] Also described herein is a turbine assembly comprising a turbine support with positive
buoyancy in water, wherein the turbine support is arranged to be anchored by an anchoring
system to a water bed and a plurality of turbine modules each with positive buoyancy
in water, wherein each turbine module is secured to the turbine support, wherein the
combined positive buoyancy of the turbine support and the turbine modules in water
has an upward force to constrain the turbine support and the turbine modules against
a downward force of an anchoring system, wherein each turbine module has a duct and
a flowing-water driveable turbine mounted in the duct, wherein the duct is for directing
water through the turbine and the turbine is for generating power from water flow.
[0083] The turbine assembly may be operable to accept any type of flowing-water driveable
turbine (i.e. axial flow turbine or cross flow turbine) once it is fitted to a turbine
module. This means that, if desired, different turbines from different manufacturers
can operate alongside each other without modification to the turbine support. This
improves flexibility in repair and maintenance and reduces cost, time and energy.
[0084] The modular turbine assembly may comprise only one turbine module. This would typically
be when the turbine module assembly is used for testing or prototyping, although there
may be other reasons, like, for example, when all but one of the turbine modules have
been detached from the turbine support for maintenance or repair or for use in a narrow
river or other size-constrained site.
[0085] Water generally flows in one direction in a river whereas tidal flow at sea generally
causes water flow in two directions. In an optional arrangement of the present invention,
the turbine is driveable by water flowing in either direction through the duct. This
has the advantage that the turbine assembly is able to harness the kinetic energy
of unidirectional river water flow or bidirectional tidal water flow at sea.
[0086] The duct reduces the effects of change in tidal flow angle, off-axis water flow and
wave interaction by straightening and aligning water flow with the axis of the turbine.
As described herein the duct defines a hollow generally cylindrical bore, wherein
the turbine is a horizontal axis turbine with a rotor co-axial with the duct, and
wherein the rotor is matched to the internal diameter of the duct.
[0087] As described herein, the duct is in fluid communication with a flared annular section
at each end of the duct and wherein each flared annular section tapers towards the
duct. Depending on the water flow direction, the down-flow flared annular section
scoops water into the duct and the other emits water. This can increase water flow
through the duct.
[0088] As described herein, the flared annular sections are mounted upon the turbine support.
This can reduce the size, weight and complexity of the turbine modules.
[0089] As described herein, boundaries between the duct and the flared annular sections
have at least one gap to promote water flow augmentation around where water flows
into the flared annular section down-flow from the duct. This reduces water eddies
by re-establishing a boundary layer connection between water flow and the diffuser
i.e. the down-flow flared annular section. A reduction in water eddies is beneficial
because it may reduce parasitic energy losses and drag. The at least one gap may be
one gap or a series of gaps or slots.
[0090] As described herein, the at least one gap is an annular gap. This can promote water
flow augmentation around the whole circumference of the diffuser i.e. the down-flow
flared annular section.
[0091] As described herein, each flared annular section has an array of transverse vanes.
The vanes help prevent ingress of marine flora, fauna and debris and guide such objects
clear of the duct. The vanes help straighten the water flowing into the turbine.
[0092] As described herein, the turbine module is streamlined to reduce interaction with
upward and downward wave motion in the water surrounding the turbine module. Wave
motion, particularly upward and downward wave motion, can put significant force on
the anchoring system of a turbine assembly and, over time, can damage or weaken the
anchoring system. This is especially so when the turbine assembly is under load of
tidal flow. Interaction with wave motion is to be reduced as much as possible by,
for example, anchoring the turbine assembly in deep water i.e. 40m of water. Streamlining
the turbine modules has the advantage of further reducing wave interaction by presenting
a decreased horizontal cross-sectional area.
[0093] As described herein, each turbine module is detachably docked with the turbine support.
All of the turbine assembly's components can be floated to an anchorage site where
they are assembled. The turbine support is submerged and permanently anchored to the
water bed. The turbine modules are submerged to detachably dock with the turbine support
where they remain until maintenance, repair or replacement is needed. In that event,
one turbine module may be floated to the water surface without need of disturbing
the rest of the turbine assembly. This saves time, energy and cost. If maintenance
or repair is unexpectedly protracted a decision to substitute the defective turbine
module can be taken quickly and efficiently. Moreover, if a substitute turbine module
is not available then the defective turbine module can be returned to harbour while
the rest of the turbine assembly continues to operate uninterrupted.
[0094] As described herein, the positive buoyancy of the turbine module is localised above
and below the duct. The turbine module can float on its side with an increased horizontal
cross-sectional area because the streamlined profile naturally lies flat upon the
water surface. This improves stability, and reduces the draft, of the turbine module
when it is being towed in water.
[0095] As described herein, the turbine module is elongate in the direction of water flow
through the duct and wherein an external surface of the turbine module has a generally
elliptical transverse cross-sectional profile. An elliptical profile is an example
of a streamlined profile that helps to reduce interaction with upward or downward
wave motion by presenting a decreased horizontal cross-sectional area.
[0096] As described herein, the turbine is removable through a removable side of the turbine
module. Complete access to the turbine in open water, and even removal of the turbine
by floating crane, may be highly beneficial in saving time, energy and cost in repair
or maintenance to the turbine module.
[0097] As described herein, each turbine module is adapted to dock with the turbine support
in a positive location arrangement. This has the advantage of automating the docking
process because the turbine module finds its own docking location as it is lowered
into the turbine support. The process may be further automated by latches to fix the
turbine module docked to the turbine support. Alternative fixing means, like, for
example, a lock or a pin may be employed.
[0098] As described herein, the turbine support is adapted to dock with three to five turbine
modules.
[0099] As described herein, the positive buoyancy of the support structure and/or the turbine
module is variable. The buoyancy of the support structure or turbine module may be
reduced to facilitate submerging, especially if a remote operated vehicle is to be
used instead of a winch. When the turbine support or the turbine module is assembled
with the turbine fully assembly the buoyancy may be increased to stiffen the anchoring
system. Variable buoyancy presents the operator with advantageous flexibility.
[0100] The turbine support may be provided separately for assembly with the modular turbine
assembly.
[0101] The turbine module may be provided separately for assembly with the modular turbine
assembly.
[0102] Also described herein is a method of assembling the modular turbine assembly in open
water which comprises the steps of: towing the turbine support to an anchorage site;
anchoring the turbine support to a water bed with an anchoring system; towing the
plurality of turbine modules to the anchorage site; submerging one of the turbine
modules to dock with the turbine support; and repeating the last step until the full
complement of turbine modules is docked with the turbine support.
[0103] The depth of the turbine assembly depends on turbine size and the conditions of the
anchoring site. The minimum submerged depth could be 1m in a river or in a sheltered
position. In certain conditions the depth may be less than 1m whereby the turbine
support is not entirely submerged. As described herein, the method of assembling a
modular turbine assembly comprising an additional step of submerging the turbine support
between steps of towing it and anchoring it.
[0104] As described herein, the last step of the method of assembling a modular turbine
assembly in open water is performed by force of a winch with at least one pull line
and wherein the winch is mounted upon the turbine support. This is a reliable way
of ensuring the turbine module docks with the turbine assembly.
[0105] Alternatively, the last step of the method of assembling a modular turbine assembly
in open water is performed by force of a remote operated submergible vehicle. This
is a suitable alternative if a winch is not fitted to the turbine support, or it is
inoperable.
[0106] As described herein, the positive buoyancy of the turbine module is reduced when
submerged by a remote access vehicle and increased after docked with the turbine support.
This makes it easier for the remote access vehicle to submerge the turbine module.
[0107] As described herein, the method of assembling a modular turbine assembly in open
water is performed with the anchoring system described above due to its inherent resistance
to heave under water flow.
[0108] Also described herein is a method of repair or maintenance to the modular turbine
assembly comprises the steps of tethering one of the turbine modules for controlled
floatation to the water surface; detaching the turbine module from the turbine support;
floating the turbine module to the water surface; and performing repair or maintenance
work upon the turbine module or submerging a substitute turbine module to dock with
the turbine support.
[0109] As described herein, the method of repair or maintenance to a modular turbine assembly
is performed by force of a winch with at least one pull line wherein the winch is
mounted upon the turbine support. This is a reliable way of ensuring the turbine module
docks with the turbine assembly.
[0110] Alternatively, the method of repair or maintenance to a modular turbine assembly
is performed by a remote operated submergible vehicle. This is a suitable alternative
if a winch is not fitted to the turbine support, or it is inoperable.
[0111] As described herein, the positive buoyancy of the turbine module is reduced when
submerged by a remote access vehicle and increased after docked with the turbine support.
This makes it easier for the remote access vehicle to submerge the turbine module.
[0112] As also described herein, the turbine support may be provided by towing the turbine
support to an anchorage site and anchoring the turbine support to a water bed with
an anchoring system, and in an arrangement this would be the anchoring system described
above.
[0113] As also described herein, the turbine module may be provided by towing the turbine
module to the anchorage site and submerging the turbine modules to dock with the turbine
support.
[0114] As also described herein, there is provided a turbine assembly comprising at least
one flowing-water driveable turbine for generating power from water flow, wherein
the turbine assembly positive buoyancy in water and the turbine assembly is arranged
to be anchored by an anchoring system to a water bed, wherein the or each turbine
is mounted in a respective duct, wherein the duct is for directing water through the
turbine and the turbine is for generating power from water flow, and wherein the duct
is in fluid communication with a flared annular section at an end of the duct and
wherein the flared annular section tapers inwardly towards the duct. The flared annular
section helps to direct water current towards the duct thereby concentrating water
flow past the turbine and increasing energy extraction. Alternatively, the flared
annular section may act as a diffuser to direct water flow from the duct.
[0115] As described herein, a boundary between the duct and the flared annular section has
at least one gap. This may reduce water eddies by re- establishing a boundary layer
connection between water flow and the duct or flared annular section down-flow of
the gap. A reduction in water eddies is beneficial because it may reduce parasitic
energy losses and drag. The at least one gap may be one gap or a series of gaps or
slots.
[0116] As described herein, the at least one gap is an annular gap. This may promote water
flow augmentation around the whole circumference of the flared annular section.
[0117] As described herein, the duct is in fluid communication with a second flared annular
section at a second opposite end of the duct and wherein the flared annular section
tapers inwardly towards the duct. The first flared annular section can act as a concentrator
of water flow through the duct while the second flared annular section can act as
a diffuser of water flow from the duct, and vice versa depending on water flow direction.
[0118] As described herein, a boundary between the duct and the second flared annular section
has at least one gap. This may reduce water eddies by re-establishing a boundary layer
connection between water flow and the duct or second flared annular section down-flow
of the gap. A reduction in water eddies is beneficial because it may reduce parasitic
energy losses and drag. The at least one gap may be one gap or a series of gaps or
slots.
[0119] As described herein, the at least one gap is an annular gap. This may promote water
flow augmentation around the whole circumference of the second flared annular section.
[0120] As described herein, the or each turbine is driveable by water flowing in either
direction through the duct. This has the advantage that the turbine assembly is able
to harness the kinetic energy of unidirectional river water flow or bidirectional
tidal water flow at sea.
[0121] As described herein, the duct defines a hollow generally cylindrical bore, wherein
the turbine is a horizontal axis turbine with a rotor co-axial with the duct, and
wherein the rotor is matched to the internal diameter of the duct.
[0122] As described herein, the positive buoyancy of the turbine assembly in water has an
upward force to constrain the turbine assembly against a downward force of an anchoring
system,
[0123] Also described herein is a method of raising a submersible platform having variable
positive buoyancy and suitable for mounting a power generating turbine thereon; wherein
the submersible platform is anchored by an anchoring system; and wherein the submersible
platform is configured to reduce the variable positive buoyancy before the anchoring
system is released. Reducing the positive buoyancy before raising the platform is
counter-intuitive, but maintaining a high level of positive buoyancy risks damaging
the platform by causing it to surface too quickly.
[0124] Further, there is provided a submersible apparatus for supporting a turbine for generating
electrical power from flowing water, comprising: four couplings, spaced apart on the
apparatus and arranged spatially so that the attitude of the apparatus can be altered;
two mooring lines wherein each mooring line is coupled to the apparatus by two tethers,
and each tether is coupled to a different one of the four couplings; an orientation
sensor configured to sense the orientation of the apparatus; and an attitude controller
coupled to the orientation sensor to alter the attitude of the apparatus in a submerged
position in a body of water to accommodate changes in the direction of flow of the
water based on the sensed orientation; wherein altering the attitude of the apparatus
comprises altering the tension in the tethers.
[0125] Further, there is provided a submersible apparatus for supporting a turbine for generating
electrical power from flowing water, the apparatus comprising: a turbine; and an orientation
controller, operable to adjust the orientation of the turbine with respect to the
apparatus and configured to change the orientation of the turbine to inhibit generation
of power from the turbine in the event that a fault is detected.
[0126] Further, there is provided a submersible apparatus for supporting a turbine for generating
electrical power from flowing water, the apparatus comprising: a turbine; and an orientation
controller, operable to adjust the orientation of the turbine with respect to the
apparatus and configured to change the orientation of the turbine to inhibit generation
of power from the turbine in the event that the speed of the water is less than a
selected threshold speed.
[0127] Further, there is provided a submersible apparatus for supporting a turbine for generating
electrical power from flowing water, the apparatus comprising: a turbine; and an orientation
controller, operable to adjust the orientation of the turbine with respect to the
apparatus and configured to change the orientation of the turbine to enable generation
of power from the turbine in the event that the speed of the water is greater than
a selected threshold speed.
[0128] Further, there is provided a remotely controllable unmanned surface vessel for controlling
a submersible, the vessel comprising: a motive unit for moving the surface vessel;
a first communication interface for communicating with a remote device, and being
coupled to the motive unit to enable the remote device to control the motive unit;
and a second communication interface for communicating with the submersible to control
the submersible, and being coupled to the first communication interface for communication
between the submersible and the remote device; wherein the surface vessel comprises
controller configured to operate the motive unit to move the surface vessel in response
to a command sent to the submersible from the remote device so that the submersible
stays within communication range of the surface vessel.
[0129] Further, there is provided a method of controlling a flowing-water driveable turbine
apparatus, the method comprising: sensing water speed; in the event that the water
speed is within a first predetermined speed range, providing a power supply to a power
deriver arranged to derive electrical power from the turbine; and in the event that
the fluid speed is not within the first predetermined speed range, not providing a
power supply to the power deriver.
[0130] Further, there is provided a flowing water driveable turbine apparatus comprising:
a water speed sensor; a power deriver for deriving electrical power from driving of
the turbine; and a controller configured to couple a power supply to the power deriver
in the event that the fluid speed is within a first predetermined speed range, and
to decouple the power supply from the power deriver in the event that the fluid speed
is not within the first predetermined speed range.
[0131] Embodiments of the turbine assembly, the anchoring system, and the anchoring cables
will now be described, by way of example only with reference to the drawings in which:
Figure 1 is a perspective view of an embodiment of the modular turbine assembly anchored
to a sea bed;
Figure 2 is a perspective view a turbine module docking with a turbine support of
the modular turbine assembly of Figure 1;
Figure 3 is a plan view of the top of the turbine support;
Figure 4 is perspective view of the turbine module;
Figure 5 is a front elevation view of the turbine module;
Figure 6 is a cross-sectional view of the turbine module with a flared annular section
at each end; and
Figure 7A to 7C show three stages of disassembling the turbine module.
Figure 8 is a perspective view of an embodiment of a turbine assembly anchored to
a sea bed by an anchoring system;
Figure 9 is a front elevation view of a modification to the turbine assembly of Figure
8 having two turbines;
Figure 10 is a front elevation view of a modification to the turbine assembly of Figure
8 having four turbines;
Figure 11 is a perspective view with cut-away of a winch for adjusting a tag line
of an anchor cable of the anchoring system of Figure 8;
Figure 12 is a vertical cross-section through a buoyancy device of the turbine assemblies
of Figures 8 to 10;
Figure 13 is a cross-section through an anchoring point of the anchoring system of
Figure 8;
Figure 14 is a perspective view of a tag line or a mooring line of an anchor cable
of the anchoring system of Figure 8;
Figure 15 is a plan view of an embodiment of a turbine assembly anchored to a sea
bed by an anchoring system;
Figure 16 is a cross-section through a buoyancy device of the turbine assembly of
Figure 15 showing the buoyancy device only containing air;
Figure 17 is a cross-section through the buoyancy device of Figure 16 showing the
buoyancy device being partially flooded with water;
Figure 18 is a side-elevation view of the turbine assembly of Figure 15;
Figure 19 is a side-elevation view of the turbine assembly of Figure 18 showing the
turbine assembly before being raised and also once it has been raised to the surface
of the water;
Figure 20 is a side-elevation view of the turbine assembly of Figure 19 showing a
diver detaching the assembly from the anchoring system;
Figures 21 and 22 show the gear mechanism used to rotate the turbine and blades;
Figure 23 shows a side-elevation view of the turbine assembly, having been detached
from the anchoring system, being towed by two vessels;
Figure 24 shows a side-elevation view of the turbine assembly where one of the tethers
in the anchoring system has broken;
Figure 25 is a plan view of the turbine assembly of Figure 24 showing the tether in
the anchoring system that has broken;
Figure 26 is a plan view of the turbine assembly of Figure 25 showing the turbines
in a fail-safe mode;
Figure 27 shows a side-elevation view of a surface vessel and a tethered submersible
alongside a turbine assembly;
Figure 28 shows a simplified system diagram of the power electronics on board the
turbine assembly;
Figure 29 shows a side-elevation view of the turbine assembly in normal operation
with tidal flow in Z direction;
Figure 30 shows a plan view of the turbine assembly of Figure 29 when the tide starts
to turn and tidal flow starts to flow in the Y direction;
Figure 31 shows a plan view of the turbine assembly of Figure 30 where the turbines
are in the middle of rotating to face into the tidal flow Y; and
Figure 32 shows a plan view of the the turbine assembly in normal operation with tidal
flow in Y direction
[0132] The submersible turbine assembly of an embodiment of the invention is provided with
sufficient fixed buoyancy to ensure that the entire assembly is slightly net positively
buoyant. This enable the turbine assembly to be buoyant during the transitional stages,
such as during installation, surfacing or retrieval.
[0133] This fixed positive buoyancy is provided by fixed buoyancy compartments that are
filled by a medium that provide sufficient displacement, for example, air, foam or
nitrogen. Buoyancy is calculated in kilograms as being the cube of volume multiplied
by the density of, in this case, sea water minus the mass of the turbine assembly:
[0134] The variable buoyancy is sized to ensure that the assembly remains at its desired
position in the water column and that the mooring lines, interchangeably referred
to as mooring line runs, remain in tension during all operational conditions. The
aim is to provide a sufficient uplift force on the assembly. The entirety of this
uplift force can be provided through the variable buoyancy of the assembly. It may
be possible to, in part or whole, substitute hydrodynamic uplift through the use of
hydrofoils in the assembly, which would reduce the amount of buoyancy needed in the
assembly to provide the uplift force. In water currents having a slower speed or lower
nominal flow velocity, however, at least some of the uplift will need to be provided
by the buoyancy. When current is not flowing relative to the assembly, the buoyancy
provides an uplift force that keeps all of the mooring lines in tension. When the
tidal steam velocities increase, the structure experiences drag. When the turbines
begin to operate, a thrust force is created. The forces acting on the turbine assembly
act to push it away from the upstream anchor point and, as the anchor point and line
lengths are fixed, this creates a moment around the anchor point and the assembly
will experience load pushing it in the direction of an arc prescribed by the mooring
line length and the anchor point. Thus, the uplift force provided must be sufficient
to overcome these forces acting to push the assembly downwards. That is to say, the
buoyancy, or uplift force, must be greater than the downward components of the combined
drag and thrust forces. In addition, the impact of the additional velocities induced
by turbulence and the accelerations, both horizontal and vertical, induced by waves
must be considered. The dynamic hydrodynamic forces induced on the structure have
both drag and added mass components. The total amount of variable buoyancy may be
sized to provide sufficient uplift to react to the combination of all of these described
loads and maintain tension in the mooring lines in operational conditions.
[0135] The submersible turbine assembly can comprise a frame sized and configured for supporting
one or more flowing water driven turbines. The assembly can be located at a predetermined
depth range below the surface of a water body, such as a sea or river, where the water
body has a bed. The frame includes at least one fixed buoyancy component and at least
one variable buoyancy component. Optionally the frame can also include one or more
hydrodynamic lift-generating surfaces. The one or more flowing water driven turbines
are expected to produce a drag force which varies with water velocity, the assembly
being arranged to be maintained in a predetermined position above the water body bed
and submerged below the expected water body surface by at least two upstream taut
mooring line runs and at least one downstream taut mooring line run, wherein the mooring
line runs are arranged to be anchored to the water body bed at respective anchor points
spaced apart from a point below the expected position of the assembly by at least
the height of the assembly above the water bed, the frame having attachment points
for said taut mooring line runs comprising upper and lower attachment points for spaced
apart attachment of upper and lower or primary and secondary mooring lines or cables
forming each mooring line run, the attachment points arranged to permit movement of
at least some of the mooring lines relative to the frame during installation and having
an arrangement for locking the mooring lines relative to the frame during use, the
assembly being arranged so that in use in a water current having a nominal flow velocity
of 3 metres per second both the upstream and downstream mooring line runs remain taut
and wherein the net upward force resulting from the fixed and variable buoyancy and
any hydrodynamic lift minus the weight of the assembly is at least 25% of the drag
on the structure and thrust produced by the at least one turbine.
[0136] Referring to Figure 1, there is shown a sea bed 2 in a region of the sea where water
flows in two directions due to tidal forces. Submerged in the water is a modular turbine
assembly 4 which is for converting the kinetic energy of the flowing water into electrical
energy and delivering it to a facility located on shore or offshore. The turbine assembly
comprises a turbine support 6 which is positively buoyant in water and which is anchored
to the sea bed by an anchoring system 8. The turbine assembly 4 has an array of five
turbine modules 10 arranged in a line in the turbine support 6. Each turbine module
10 is positively buoyant in water. Each turbine module 10 is detachably docked to
the turbine support 6. The combined positive buoyancy of the turbine support 6 and
the five turbine modules 10 has an upward force which constrains against the downward
force of an anchoring system 8.
[0137] As the turbine assembly of this embodiment is anchored at sea, a double-headed arrow
T shows both directions in which the tidal forces cause the water to flow. The modular
turbine assembly 4 is orientated with the array of five turbine modules 10 generally
in line with arrow T so that as much water as possible flows through the turbine modules
in a straight path.
[0138] The turbine assembly 4 is described as modular because the turbine modules 10 are
interchangeable with each other and are docked to the in the support structure 6 in
the same way.
[0139] Referring to Figure 2, there is shown a turbine module 10' being pulled downward
by a winch on the turbine support with two pull lines 12. The turbine module 10' docks
with the turbine support 6. Once docked with the support structure, the turbine module
10' is anchored to the sea bed by the anchoring system unless, or until, at some time
in the future the turbine module 10' is detached for maintenance, repair or replacement.
[0140] If, or when, maintenance, repair or replacement is required, the turbine module 10'
is detached from the turbine support and allowed to float under its own inherent buoyancy
in water to the surface. The assent of the turbine module 10' is controlled by the
winch with two pull lines 12.
[0141] Alternatively, the winch with two pull lines can be substituted by a remote operated
vehicle to perform the task of submerging the turbine module to dock with the turbine
support. The remote operated vehicle can perform the task of controlled floatation
of the turbine module to the surface too.
[0142] Referring to Figure 3, the turbine support 6 comprises a frame 14. The frame can
be made of any material strong enough to support the turbine modules (i.e. steel,
aluminium, fibre reinforced concrete, inflated material or composite). The frame has
elements that are filled with buoyant material, or that are attached to buoyant material,
to provide the positive buoyancy of the turbine support. The positive buoyancy may
be adjusted by means of compressed air or buoyant gel or by another medium pumped
from the surface or supplied by sub-sea reservoir. The positive buoyancy of the turbine
support 6 is enough to be towed to the anchoring site.
[0143] The frame 14 is divided into five turbine module docking bays 16. Each docking bay
16 is accessible through the top of the frame to receive a respective turbine module
10. Each docking bay has a pair of flared annular sections 20a, 20b connected to the
frame 14 of the turbine support 6. One flared annular section is located at each end
of the docking bay. Each flared annular section tapers towards the docking bay.
[0144] Referring to Figures 4 and 5, the turbine module 10 has a major body shell 22 and
a minor body shell 24 joined to form a duct 26 which defines a hollow generally cylindrical
bore. An external surface 27 of the joined minor and major body shells has a generally
elliptical profile transverse the cylindrical bore of the duct. The body shells 22,
24 are filled with buoyant material (i.e. a fluid, solid or a combination of both),
or are attached to buoyant material, to provide the positive buoyancy of the turbine
module. The positive buoyancy of each turbine module 10 is enough to be towed to an
anchoring site.
[0145] The turbine module 10 has a water-driveable horizontal axis turbine 28 mounted upon
a bracket 30 inside the duct. The turbine has a rotor 32 co-axial with the duct. The
rotor is matched to the diameter of the duct. The duct shields the turbine from turbulence
caused by adjacent turbines so that the array of five turbine modules may be closely
spaced. The turbine is driveable by water flowing in either direction through the
duct and generates electrical power.
[0146] Returning to Figure 2 in more detail, each turbine module 10 is docked with a respective
docking bay 16 in a complementary locating arrangement which automatically orientates
a major axis 18 of the elliptical profile of the external surface 27 in a generally
upright position in the turbine support 6 where the turbine module is locked in place
by a locking mechanism. This reduces the horizontal cross-sectional area of the turbine
module. As a result, the turbine module is streamlined to reduce interaction with
upward or downward wave motion in the surrounding water.
[0147] Electrical connections between the turbine modules 10 and the turbine support 6 are
made before or after docking. The electrical power generated by the turbines varies
with water flow rate. Each turbine module has electrical power equipment (not shown)
for conditioning the electrical power generated by the turbines. The turbine support
has electrical power management equipment (not shown) for combining the conditioned
electrical power from the five turbine modules. The turbine support's electrical power
management equipment includes a step-up transformer (not shown) for transmission of
the generated electrical power to a shore, or offshore, facility via a power cable
40. A communication cable 42 from the turbine assembly accompanies the power cable.
[0148] Referring to Figure 6, there is shown the duct 26 in fluid communication with the
pair of flared annular sections 20a, 20b when one of the turbine modules 10 is docked
in one of the docking bays 16 (not shown) of the turbine support 6 (not shown). The
annular sections are suited for bi-directional water flow. Single headed arrow T indicates
a direction of water flow which results in the up-flow annular section 20a performing
the role of concentrator to scoop water into the duct and the down-flow annular section
20b performing the role of diffuser to emit water from the duct. This situation will
be reversed when the tide changes and water flows through the duct in the opposite
direction and arrow T is reversed (i.e. annular section 20b becomes the concentrator
and annular section 20a becomes the diffuser).
[0149] The geometry of the annular sections 20a, 20b is matched to the water flow requirements
of the turbine. The annular sections may be made of steel, aluminium, fibre reinforced
concrete, inflated material or composite. The annular sections are connected may contribute
the positive buoyancy of the turbine support.
[0150] The boundaries between the duct 26 and the flared annular sections 20a, 20b each
have an annular gap 34a, 34b. The gaps enable water flowing outside the turbine module
to enter the diffuser (i.e. the down-flow annular section 20b in this example) by
venturi effect. This promotes water flow augmentation which reduces water eddies by
re-establishing a boundary layer connection between water flow and the diffuser. A
reduction in water eddies is beneficial because it reduces parasitic energy losses
and drag.
[0151] The ends of the flared annular sections 20a, 20b facing away from the duct 26 are
each equipped with an array of transverse vanes 36a, 36b. The vanes help prevent ingress
of debris into the duct and help straighten the water flowing into the turbine 28.
The vanes induce a rotational flow into the water flow to increase the energy extraction
of the turbine.
[0152] Referring to Figure 7A, the positive buoyancy of the turbine module 10 is localised
about the major axis 18 of the elliptical external surface 27. As a result, the turbine
module tends to float on the water surface with an increased horizontal cross-sectional
area. This improves stability, and reduces the draft, of the turbine module when it
is being towed at sea.
[0153] Referring to Figure 7B, the major body shell 22 has positive buoyancy to enable removal
of the minor body shell 24 while the major body shell and the turbine 28 remain afloat.
Removal of the minor body shell allows complete access to the turbine, and even removal
of the turbine by floating crane, for the purpose of repair or maintenance to the
turbine module, as is shown by Figure 7C.
[0154] Returning in more detail to Figure 1, the anchoring system 8 comprises eight pairs
of anchoring cables 44a, 46a - 44h, 46h. Each pair of anchoring cables includes an
upper anchoring cable 44 and a lower anchoring cable 46. Two pairs of anchoring cables
are fixed to each corner edge of the frame 14 of the turbine support 6 (i.e. the upper
anchoring cable of each pair is fixed to the corner edge above where the lower anchoring
cable of each pair is fixed to the corner edge). The other ends of each pair of anchoring
cables are permanently fixed to a respective anchor point 48a - 48h on the sea bed.
[0155] The anchoring cables 44a, 46a - 44h, 46h of each pair of anchoring cables converge
from the frame 14 of the turbine support 6 to their respective anchoring points 48a
- 48h. The mean angle of inclination of the anchoring cables of each pair of anchoring
cables with respect to the horizontal is approximately 30 degrees.
[0156] The anchor points 48a - 48h are arranged about the turbine support 6 to suit the
sea bed topography and to maintain the turbine support in a generally horizontal position.
The anchor points cover a footprint greater in width and in length than the turbine
support.
[0157] The upward force of the combined positive buoyancy of the turbine support 6 and the
five turbine modules 10 cause tensile forces along the full length of the anchoring
cables 44a, 46a - 44h, 46h.
[0158] The anchoring cables may be (high performance) synthetic rope, steel / wire rope,
chain, solid metallic rod or solid composite rod.
[0159] The anchoring cables are equipped with vortex suppressants to reduce their hydrodynamic
drag and reduce any vibration caused by water flow. For example, a vortex suppression
system may be fibre or tape strands incorporated or attached to the anchoring cables.
The fibre or tape strands stream with the water flow to form a fairing, or a hydrofoil.
Rotating faired sections which fit over the anchoring cables and align with the water
flow, spiral sections either fitted to or incorporated into the structure of the anchoring
cable, or other proprietary vortex suppression systems are also suitable.
[0160] Returning to Figure 2, the power cable 40 and the communication cable 42 from the
turbine assembly are incorporated within the upper anchoring cable 44a.
[0161] The modular turbine assembly is assembled at sea by towing the turbine support to
an anchorage site, submerging it, and anchoring it to the sea bed with the anchoring
system where it remains permanently. The five turbine modules are towed to the anchorage
and submerged, each one in turn, to dock with the turbine support.
[0162] The following features each can be provided independently or may be combined.
[0163] A tethered sub-sea installation base which, when populated with devices, in itself
comprises a small array of horizontal axis Tidal Energy Convertors (TECs). The base
is for use at deep water sites (over 40msw) and enables the TECs to be positioned
at the optimum depth dictated by the compromise between power output (strongest current
found close to the surface, i.e. that having the fastest nominal flow velocity) and
adverse structural and flow influences from wave interaction. Alternatively, the base
may be used at shallower sites where it is submerged very close to, or even slightly
protruding above (provided the TECs are submerged), the water surface.
[0164] As an integral part of the design a method is disclosed of installing and retrieving
the TECs using buoyant modules into which individual TECs are loaded. The loaded modules
are then towed to site and connected to the sub-sea base electrically and via a pull
in line. The module is pulled sub-sea by the pull in line and interfaces with and
locks into the sub-sea base.
[0165] An alternative to the above method, the buoyant modules may be driven to and retrieved
from the PMSS by means of a Remote Operated Vehicle (ROV) specifically designed for
the purpose and having the required thrust capability. This may include the use of
variable buoyancy within the buoyant module to reduce the quantity of thrust required
to drive the module subsea.
[0166] The turbine support may be a permanently installed buoyant subsea structure PMSS
comprising:
• Structural space frame which may be of steel, aluminium or composite construction
- the elements of which may be sealed to form pressure vessels, or may be filled with,
surrounded by, or have attached buoyant material (including air or other gas) providing
all or part of the buoyancy required to support the structure.
• 'Conical' diffuser and concentrator sections - the precise geometry of the concentrator
and diffuser can be matched to the flow requirements of the TEC.
• The diffuser and concentrator sections can be suited to bi-directional flow
• The diffuser and concentrator sections can incorporate 'slots' to enable flow augmentation
to re-establish boundary layer connection within the diffuser.
• The diffuser and concentrator sections may be constructed from steel, aluminium,
fibre reinforced concrete, inflated material (i.e. 'hyperlon' or similar), composite
(i.e. glass or other fibre reinforced plastic).
• The diffuser and concentrator sections may contribute to the buoyancy of the PMSS
• Buoyancy of the PMSS may be adjustable by means of compressed air or buoyant gel
or other medium pumped from the surface or supplied from a subsea reservoir.
• Step up transformer for transmission of generated electrical power to shore or offshore
processing facility via power cable.
• Power conditioning and switching equipment as required to combine and transmit the
output of one or more tidal energy convertors as electrical power.
[0167] The anchoring system is a tension spread mooring system (TSM)
- Sea-bed fixing points which may be drag anchors; gravity anchors; suction piles; pinned
template structures; attachment to sub-sea geographical features;
- Tension members which may be high performance synthetic rope such as UHMwPE (i.e.
dyneema); steel / wire rope; chain; solid metallic rod (i.e. nitronic 50; 17-4pH;
316 stainless steel etc.)
- Vortex Induced Vibration (VIV) suppression system which may be fibre or tape strands
incorporated or attached to the tension member which streams with the flow to form
a fairing ('hairy' or 'ribbon' fairing); Rotating faired sections which fit over the
tension member and align with the flow; Spiral sections either fitted to or incorporated
into the structure of the tension member, or other proprietary vortex suppression
system.
- Power transmission cable incorporating power conductors and communications (i.e. fibre
optic or conventional signal pair conductors)
- The power transmission and communication cables may be incorporated into one or more
of the tension members (i.e. the structural cable casing may act as tension member(s)).
[0168] The turbine module is a buoyant module (BM)
- Parallel annular duct matched to TEC to reduce the effects of off axis flow and wave
interaction by straightening and aligning current flow with the TEC axis.
- Once installed the BM Integrates with the 'conical' diffuser and concentrator sections
(which form part of the PMSS) to enhance performance over that achievable in open
ocean conditions.
- BM's installed into the subsea structure by sub-sea pull in lines, buoyancy control
or a combination of the two.
- The BM has an elliptical (or otherwise non-circular) distribution of volume to reduce
the horizontal area presented to wave motions, and to give stability when on the surface.
- The BM can contain power conditioning equipment as required for each individual TEC
to enable the power produced to be fed to the centralised step up transformer for
onward transmission.
- The BM can be split to allow installation of the TEC by means of overhead crane. This
minimises the crane capacity required.
Power for sub-sea operations:
[0169] Power to drive the subsea winches may be provided by equipment permanently or temporarily
fitted to the sub-sea structure, or may be provided by means of an umbilical connection
from a surface ship, or by specially equipped Remote Operated Vehicle (ROV).
[0170] Protection against object ingress. Vanes on the concentrator and diffuser may provide
some or all of the following functions:
- Prevent the ingress of marine fauna, flora and flotsam/debris
- Guide objects clear of the duct and sub-sea structure
- Further straighten the flow into the turbine blades
- Induce counter rotational flow into the water stream to increase the energy extraction
potential.
[0171] The vanes are not a fundamental part of the design but may have significant efficiency
benefits if considered as part of the turbine design as it may allow significantly
higher rotor speeds and therefore lighter, lower cost generators. The design of the
vanes may simply look similar to two traditional 'cow catchers', mirrored and joined
on the centre-line to form an inlet guard - one such unit at each end to catch and
guide any objects clear of the inlet to the turbine duct.
[0172] Referring to Figure 8, there is shown a sea bed 102 in a region of the sea where
water flows in two directions due to tidal forces. Submerged in the water is a turbine
assembly 104 which is for converting the kinetic energy of the flowing water into
electrical energy and delivering it to a facility located on shore or offshore. The
turbine assembly comprises a turbine support 106 which is anchored to the sea bed
by an anchoring system 108. The turbine assembly 104 has a single turbine 110 secured
to the turbine support 106. The turbine support 106 comprises a frame 106a and a pair
of buoyancy devices 107 located at opposite ends of the frame 106a for stability.
[0173] The turbine 110 is secured to the frame 106a between the buoyancy devices 107. The
turbine is located approximately midway between the buoyancy devices to minimise instability
about the X axis of the turbine assembly which passes through the central axis of
the turbine. Optionally, the turbine 110 may be detachable and interchangeable with
other turbines.
[0174] The turbine support 106 has positive buoyancy in water which is variable by virtue
of the buoyancy devices 107, as is explained in more detail below. Optionally, the
turbine 110 may have variable positive buoyancy in water. The combined positive buoyancy
of the turbine assembly 104 in water has an upward force which constrains against
the downward force of the anchoring system 108.
[0175] As the turbine assembly of this embodiment is anchored at sea, a double-headed arrow
T shows both directions in which the tidal forces cause the water to flow. The turbine
assembly 104 is orientated with axis X through the turbine 110 generally in line with
arrow T so that as much water as possible flows through the turbine in a straight
path. The buoyancy devices 107 are streamlined and elongate in the direction of axis
X of the turbine assembly to minimise hydrodynamic resistance to water current flowing
in line with arrow T.
[0176] The transverse cross section of the buoyancy devices are shaped so as to reduce wave
induced motion. The upper part of the buoyancy devices are streamlined, and the lower
parts are bluff shaped so that, in addition to any resistance to motion provided by
the buoyancy or the anchoring, the hydrodynamic drag force associated with moving
the buoyancy device downward through the water is greater than the hydrodynamic drag
force associated with moving the buoyancy device upward through the water. Thus, the
buoyancy device is shaped so that, when in use it is positioned on the turbine assembly,
it is broader about its base than about its apex, and the upward facing surfaces of
the buoyancy device are more closely aligned with the vertical than the downward facing
surfaces.
[0177] Referring to Figure 9, there is shown a first alternative turbine assembly 204 which
comprises a turbine support 206 similar to the turbine support 106 mentioned above
albeit having two turbines 110 secured to the turbine support 206. The turbine support
206 comprises a frame 206a and three buoyancy devices 107, two located at opposite
ends of the frame 206a for stability and a third buoyancy device 107 in the middle
of the frame 206a for additional buoyancy. The turbines 110 are secured to the frame,
one turbine in each gap between the buoyancy tanks. The turbines are located approximately
midway between the buoyancy devices to minimise instability about the X axis.
[0178] Referring to Figure 10, there is shown a second alternative turbine assembly 304
which comprises a turbine support 306 similar to the turbine supports 106, 206 mentioned
above albeit having four turbines 110 secured to the turbine support 306. The turbine
support 306 comprises a frame 306a and three buoyancy devices 107, two located at
opposite ends of the frame 306a for stability and a third buoyancy device 107 in the
middle of the frame 306a for additional buoyancy. The turbines 110 are secured to
the frame, two turbines in each gap between the buoyancy tanks. The centre of gravity
of each pair of turbines is located approximately midway between the buoyancy devices
to minimise instability about the X axis.
[0179] Referring to Figures 8 to 10, the frame 106a, 206a, 306a of the turbine support can
be made of any material strong enough to support the turbine or turbines 110 (i.e.
steel, aluminium, fibre reinforced concrete, inflated material or composite).
[0180] Referring to Figure 11, the buoyancy devices 107 are secured to the frame 1006a,
206a, 306a. Each buoyancy device comprises a fixed buoyant material 112, such as foam,
at opposite ends of the buoyancy device and a ballast tank 114 located in between
the fixed buoyant material. The buoyancy of the turbine support may be adjusted by
filling the ballast tank with water to reduce positive buoyancy. Reduced positive
buoyancy may be desirable to reduce the force required to submerge the turbine assembly
during its installation at a target location. The buoyancy of the turbine support
may also be adjusted by emptying the water from the ballast tank and replacing it
with air to increase positive buoyancy. Increased positive buoyancy may be desirable
to make it easier to tow the turbine assembly to the anchoring site. The air may be
compressed air stored on the turbine support. Alternatively, buoyant gel may be used
or by another buoyancy medium pumped from the surface or supplied by sub-sea reservoir.
The ballast tank 114 is located midway between equal amounts of fixed buoyant material
112 to minimise instability about the Y axis of the turbine assembly when the buoyancy
of the ballast tank is being varied.
[0181] Figure 11 shows a cross-section through one buoyancy device 107 secured at an upper
and lower attachment points, at an end of the turbine support 106, 206, 306 and it
shows parts of the frame 106a, 206a, 306a equipped with a tag line winch 116 and a
mooring line winch 118. There is shown a tag line winch at each nose end 120 of the
buoyancy device. Each nose end 120 is partially formed of fixed buoyant material 112
and has a fairing which covers the tag line winch. There is also shown a mooring line
winch underneath the water ballast tank. The purpose of the winches is explained in
more detail below. The upper and lower attachment points can be located at any suitable
place on the body of the turbine assembly.
[0182] Returning to Figure 8, the turbine 110 has an elongate generally cylindrical body
shell 122 coaxial with axis X to minimise hydrodynamic drag. As mentioned above, the
turbine may have positive buoyancy and, if so, the body shell 122 is filled with buoyant
material (i.e. a fluid, solid or a combination of both), or is attached to buoyant
material.
[0183] The turbine 110 is a water-driveable horizontal axis turbine with a rotor 132 having
two rotor blades. The rotor may have three of more rotor blades. The rotor diameter
is typically 16m for a 1MW turbine although the rotor diameter may range from 2 metres
for a 50kW turbine, 6 metres for a 200kW turbine and 20 metres for a 2MW turbine.
The turbine shown in Figure 1 is not ducted, although as duct may be fitted around
the rotor to shield the turbine from turbulence caused by any adjacent turbines or
wave action and to increase efficiency of energy extraction from the water current.
For example, the rotor 132 of the turbine 110 may be located in a duct in fluid communication
with a concentrator at one end of the duct and a diffuser at the other end of the
duct, like the duct 26 and the pair of flared annular sections 20a, 20b shown in Figure
6. The turbine 110 is driveable by water flowing in either direction through the rotor
132 and generates electrical power.
[0184] Electrical connections between the turbine 110 and the turbine support 106, 206,
306 are made when the turbine is secured to the turbine support. The electrical power
generated by the turbines varies with water flow rate. Each turbine has electrical
power equipment (not shown) for conditioning the electrical power generated by the
turbines. The turbine support has electrical power management equipment (not shown)
for combining the conditioned electrical power from the turbine or turbines. The turbine
support's electrical power management equipment includes a step-up transformer (not
shown) for transmission of the generated electrical power to a shore, or offshore,
facility via a power cable 140. A communication cable 142 from the turbine assembly
accompanies the power cable.
[0185] The anchoring system 108 comprises four anchoring cables 144a - 144d. Each anchoring
cables comprises a mooring line 145a - 145d and a tag line 146a - 146d branching from
the mooring line at an intermediate point along the mooring line. One end of the mooring
line of each anchoring cable is connected to its nearest lower corner of the turbine
frame 106a, 206a, 306a. The mooring lines are connected below the centre of buoyancy
of the turbine assembly to maintain stable pitch and roll attitude. The tag line of
each anchoring cable is connected to the turbine frame at the nearest nose end 120
of its nearest buoyancy device 106a, 206a, 306a (i.e. the tag line of each anchoring
cable is connected to a corner of the frame end approximately above the corner of
the frame where the mooring line is connected in the direction of upward force of
the positive buoyancy). As such, an end of the tag and mooring lines of an anchoring
cable are connected to each corner of the frame. The tag lines need not be attached
at the corners, and may be arranged at some point beneath the buoyancy device away
from its nose or tail.
[0186] The other opposite end of each mooring line is permanently attached to a respective
anchor point 148a - 148d on the sea bed. The power cable 140 and the communication
cable 142 from the turbine assembly are incorporated within the mooring line 145b.
[0187] The anchoring cables 144a - 144d diverge outwardly from turbine support to the water
bed. The mooring lines 145a - 145d under tension form the edges of a substantially
pyramidal shape on the sea bed. The mean angle of inclination α of the mooring line
of each anchoring cable with respect to the horizontal is approximately 25 degrees.
The mean angle of inclination β of the tag line of each anchoring cable with respect
to the mooring line is approximately 15 degrees.
[0188] The anchor points 148a - 148d are arranged about the turbine support 106, 206, 306
to suit the sea bed topography and to maintain the turbine support in a generally
horizontal position. The footprint of the anchoring system upon the sea bed, as defined
by where the mooring lines of the anchoring cables are attached to the anchoring points
148a - 148d, is greater in width and in length than the turbine support. The enlarged
footprint improves the stability of the turbine assembly.
[0189] The upward force of the positive buoyancy of the turbine assembly 104, 204, 304 causes
tensile forces along the full length of the anchoring cables 144a - 144d. The turbine
assembly 104, 204, 304 anchored by the anchoring system 108 typically has an operational
depth in the top third of water column where power extraction from water current,
i.e. the nominal flow velocity of the water, is optimal. The turbine assembly may
be arranged to float submerged at a depth of at least 5m or 7.5m beneath the surface,
and/or to float submerged at a depth selected so that the vertical wave particle velocities
do not exceed 1 metre per second more than 5% of the time.
[0190] This is unlike traditional anchoring systems, such as gravity anchors or columns
driven into sea bed, which have an operation depth in the bottom third of water column
where power extraction from water current is sub-optimal because the nominal flow
velocity of the water currents are slower down there.
[0191] The fixed buoyant material 112 of the buoyancy devices 107 has sufficient positive
buoyancy in water at the operational depth with zero water current and wave loading
when the turbine assembly 104, 204, 304 is anchored to the sea bed by the anchoring
system 108. A variable component of positive buoyancy is additionally required to
provide sufficient upward force to counteract the drag moment around the anchor points
148a - 148d created by longitudinal drag caused by current flow and longitudinal and
horizontal wave particle velocities. Water current nominal flow velocity, interchangeably
referred to as water current speed, can vary between 0m/s (calm) and 8m/s (storm conditions)
and the optimal water current speed for peak power output from the turbines 110 is
about 2.5m/s. The mean water current speed at any particular site depends on factors
such as depth of water column, location of turbine assembly and the bathymetry of
the sea bed. The variable component of positive buoyancy provided by the ballast tanks
114 can either be varied (i.e. by emptying of ballast tanks of water and filling them
with air) upon installation of the turbine assembly at site and then be constant for
its operational life or it can be varied during its operational life if required.
Additionally or alternatively, the variable positive buoyancy can be supplemented
throughout the tidal cycle by hydrodynamic upward force. Hydrodynamic upward force
may be provided by hydrofoils, or fins, secured to the turbine support 106a, 206a,
306a. Referring to Figures 8 to 10, there is shown hydrofoils 150 protruding outwardly
from the ballast tanks 114. The positive buoyancy and hydrodynamic upward force required
to counteract the drag moment around the anchoring points is proportional to the maximum
water current speed experienced by the turbine assembly. The hydrofoils 150 are shaped
to increase upward force with increasing water current and, in doing so, provide increasing
counteraction to the drag moment around the anchor points caused by increasing longitudinal
drag and maintain the turbine assembly at the desired elevation above the sea bed.
[0192] In normal operating conditions, excursion of the turbine assembly may be about +/-
2 metres in both the horizontal and vertical planes. In storm conditions, excursion
of the turbine assembly may be about +/- 10 metres in both the horizontal and vertical
planes.
[0193] In practice, we have found that the proportion of the variable upward force divided
by the total upward force (fixed and variable) of the turbine assembly should be 10%
to 20% of the figure (expressed as metres/second) of the maximum current flow speed
in line with the turbine assembly. Likewise, we have found that the proportion of
the variable upward force divided by the total weight of the turbine assembly should
be 20% to 30% of the figure (expressed as metres/second) maximum current flow speed
in line with the turbine assembly.
[0194] The tag lines 146a - 146d and the mooring lines 145a - 145d of the anchoring cables
may be ropes made of nylon, polypropylene and/or high performance polyethylene materials
or the anchoring cables may be steel / wire rope, chain, solid metallic rod or solid
composite rod. The tag lines are made of different material to the mooring lines and
the tag lines have a greater elasticity than the mooring lines. Thus, the mooring
lines are to constrain the turbine assembly against the upward force of the positive
buoyancy of the turbine assembly. The tag lines are to provide directional support
to counteract pitch, roll or yaw movement of the turbine assembly 104, 204, 304 about
the X, Y and Z axes. The tag lines have integral resistance to shock in the event
of sudden movement of the turbine assembly. Additional resistance to shock may be
provided by dampers connected in series or in parallel with one or more of the tag
lines.
[0195] Returning to Figure 11, each tag line 146a - 146d is connected to a respective tag
line winch 116 and each mooring line 145a - 145d is connected to a respective mooring
line winch 118. The winches 116, 118 are operable to vary the length of the tag lines
and the mooring lines. The winches are used to submerge the turbine assembly 104,
204, 304 from the water surface to its target operational depth as is explained in
more detail below. Additionally, the winches may be used to adjust the orientation
of the turbine assembly about the X, Y and Z axes and, in doing so, orientate it in
the optimal direction when anchored to the sea bed. The winches are externally activated,
typically by a remote operated vehicle, although the winches may be activated by their
own electric motor. Each winch is ratcheted to lock it against unintentional release
of the tag lines and mooring lines under tension. The frame 106a, 206a, 306a of the
turbine assembly is equipped with the tag line and mooring line winches, but the skilled
person will understand that the winches may be installed on the anchoring system,
such as near or at the anchoring points 148a - 148d.
[0196] In addition, the winches may be arranged in only one of the buoyancy devices, with
points of attachment for pull lines distributed about the turbine assembly. In this
way
[0197] Referring to Figure 14, there is shown the mooring line 145a of anchoring cable 144a
where it is connected to the anchoring point 148a. The anchoring point protrudes from
in a hole drilled 15 metres into the sea bed at an angle α of approximately 25 degrees
to the horizontal. The drilling operation may be performed by a remote operate vehicle
deployed upon the sea bed. Approximately the bottom ten metres of the hole are in
bed rock 150 and approximately the top five metres of the hole (including the open
mouth of the hole) are in weathered rock 152. The diameter of the hole is approximately
0.25m into which a single or multiple tendon anchor 154a is installed and grouted.
The anchoring point 148a is on the exposed end of the anchor 154a and the mooring
line is connected thereto. Once the grout has hardened tension may be applied to the
mooring line.
[0198] The mooring lines 145a - 145d and the tag lines 146a - 146d of the anchoring cables
are equipped with vortex suppressants to reduce their hydrodynamic drag and reduce
any vibration caused by water flowing past them. The vortex suppressant comprises
a helical protrusion 156 arranged about the circumference of each line and woven into
or bonded to the strands of the rope material used to make the line. The helical protrusion
has a pitch 158 of approximately twelve times the diameter 160 of the line, although
a pitch 158 falling within the range of four to sixteen times the diameter 160 of
the line can be used. The helical protrusion has an outer diameter 162 of approximately
150% of the diameter of the line, although an outer diameter 162 falling within the
range of 110% to 200% times the diameter 160 of the line can be used.
[0199] The helical protrusion 156 is applied to the rope of the mooring line 145a - 145d
or tag line 146a - 146d by one or more of the following methods:
- a) Arranging the weave of the material used to make the rope so that a helix is generated
which is more pronounced than the other windings, and displays the characteristics
of the pitch ratio described above;
- b) Additional materials may be added during the production of the rope to bulk out
the rope to form the helix. The bulking material may the same material as the rope
or a rigid section of thermoplastic material pre-formed as a helix and bonded to the
rope; and/or
- c) An outside cover that is either wrapped or whipped around the rope with parts woven
in for continuity at various points. Materials could be the same as the core rope
or the others listed above.
[0200] Other possible vortex suppressants include fibre or tape strands incorporated or
attached to the anchoring cables. The fibre or tape strands stream with the water
flow to form a fairing, or a hydrofoil. Rotating faired sections which fit over the
anchoring cables and align with the water flow, spiral sections either fitted to or
incorporated into the structure of the anchoring cable, or other proprietary vortex
suppression systems are also suitable.
[0201] The floating turbine assembly 104, 204, 304 is initially assembled in harbour whence
it is towed to an anchorage site where four anchoring points 148a - 148d have been
fixed to the sea bed. The mooring lines 145a - 145d and the tag lines 146a - 146d
of the anchoring cables 144a - 144d are unwound from their respective winches 116,
118 and are submerged towards the sea bed. The free ends of the mooring lines are
fixed to respective anchoring points. The variable positive buoyancy is reduced by
filling the ballast tanks 114 with water. The winches are turned slowly to wind up
the tag lines and mooring lines. The winches are operated by remote operated vehicle.
The turbine assembly is steadily submerged to its operational depth. The ballast tanks
are re-filled with air upon arrival at the operational depth. This increases positive
buoyancy so that the turbine assembly is anchored to the sea bed by the anchoring
system 108 with tensile forces in the mooring lines and the tag lines.
[0202] Referring to Figure 15, there is shown a floating turbine assembly 104 with two turbines
110 having rotors 132 oriented facing the surface of the water, i.e. in an upright
orientation, and three buoyancy devices 107. The floating turbine assembly 104 is
tethered using an anchoring system having four mooring lines 145a, b, c & d attached
at four anchoring points 148a, b, c & d fixed to the sea bed. The floating turbine
assembly 104 also has a buoy 1004 connected to the central buoyancy device 107 with
buoy line 1005. The turbines 110 and rotors 132 are in an upright orientation to prevent
them from turning in the current and generating power, and in this position mechanical
braking is also provided by the gearing in the turbines.
[0203] Referring briefly to Figure 18, support vessels 1003 and 1002 are shown arriving
at the site where the floating turbine assembly 104 is installed. The vessels or personnel
on board the vessels check the conditions and whether there is any local traffic before
beginning the retrieval of the floating turbine assembly 104. At this point, if the
turbines 110 and rotors 132 are not in an upright orientation (and are still generating
power), the turbines 110 and rotors 132 are oriented to face the surface of the water,
i.e. upright, and power generation is stopped.
[0204] Referring now to Figure 16, the buoyancy devices 107 are shown with ballast tanks
1001. The rotor 132 is visible in an upright orientation. The ballast tanks 1001 in
the buoyancy devices 107 are filled with air for normal operation, as described above,
such that the floating turbine assembly 104 has sufficient variable positive buoyancy
so the assembly 104 is anchored to the sea bed with a net positive buoyancy that causes
the mooring lines in the anchoring system 108 to be under tension with tensile forces
in all of the mooring lines.
[0205] Referring to Figure 17, the variable positive buoyancy of the floating turbine assembly
104 is reduced by filling the ballast tanks 1001 in the buoyancy devices 107 such
that the tanks 1001 are fully flooded with water. The floating turbine assembly 104
is now ready to be raised to the surface.
[0206] Referring back to Figure 18, the respective winches release the upstream upper and
lower lines 145b,c, such that the floating turbine assembly 104 rises to the surface
of the water as shown in Figure 19. Figure 19 shows arrow X, which indicates the direction
of movement of the floating turbine assembly 104 to the water surface. The water in
the ballast tanks 1001 can be expelled if required, for example using compressed air
cylinders located on one of the vessels 1002, 1003.
[0207] Figure 20 shows a diver 1007 diving from vessel 1002 to repair the broken line and
replace it as required.
[0208] Figures 21 and 22 show more detail on the rotation mechanism for the turbines 110
and blades 132. Figure 21 shows a rack and pinion system at three different positions
corresponding to the turbine being rotated to either 0 degrees, 90 degrees or 180
degrees. Figure 22 shows the rack and pinion system as it would be provided in the
assembly. The arrangement is biased such that the turbine will default to the 90 degrees
orientation, such that the turbine 110 and blades 132 will face upwards. The rack
is mounted on a carriage, wherein the rack and carriage move on a track which is controlled
by a first ram, the first ram also being mounted on a carriage, wherein the first
ram and carriage is moved by a second ram which is fixed in position on the structure.
[0209] Figure 23 shows the floating turbine assembly 104 being towed by vessels 1003 and
1002 once detached from the sea bed. Towing bridles are secured between the floating
turbine assembly 104 and the vessels 1003 and 1002.
[0210] Referring to Figure 24, there is shown a floating turbine assembly 104 with two turbines
110 having rotors 132 oriented to generate power, i.e. facing into the direction of
the current, and three buoyancy devices 107. The floating turbine assembly 104 is
tethered using an anchoring system having four mooring lines 145a, b, c & d attached
at four anchoring points 148a, b, c & d fixed to the sea bed but one of these mooring
lines 145a is shown as having broken. The floating turbine assembly 104 also has a
buoy 1004 connected to the central buoyancy device 107 with buoy line 1005. Figure
25 shows the same floating turbine assembly 104 from above, again showing mooring
line 145a is broken.
[0211] Referring now to Figure 26, the broken mooring line 145a is an upstream line as,
of the mooring lines 145, the upstream lines have the highest probability of breaking
since they undergo the most load. The turbines 110 and blades 132 are rotated to an
upright orientation to stall the turbines 110, stop the generation of power and remove
the thrust forces, reducing the overall load that the remaining mooring lines must
react against. The turbines 110 are actuated by hydraulic rams that are biased into
the upright orientation, requiring power to be applied in order to move the turbines
110 and blades 132 to face into the current. Thus if there is, for example, a power
failure on board the floating turbine assembly 104 then the turbines 110 and blades
132 will move into an upright orientation. The floating turbine assembly 104 is then
brought to the surface to enable support vessels and a diver to replace the broken
mooring line 145a and re-submerge the floating turbine assembly 104.
[0212] In an alternative embodiment, the buoyancy tanks 1001 (see Figures 16 & 17) can be
flooded to reduce the loads on the mooring lines 145.
[0213] Figure 27 shows another aspect of the present invention where there are provided
a remotely operated surface vessel 1011 and a submersible 1012. The submersible 1012
is tethered to the surface vessel 1011 so that the tether 1013 can be used to supply
power to the submersible 1012 from the surface vessel 1011. The tether 1013 also provides
a route for communication between the vessel 1011 and the submersible 1012 by incorporating
a suitable cable, e.g. optical fibre, into the tether 1013 to allow direct communication.
The submersible 1012 can be used to install anchor points into the sea bed 150 as
for example in Figure 13. An advantage of using a combined unmanned surface vessel
and tethered submersible is that the process of installing anchor points and other
installation and maintenance work underwater then reduces or removes the need for
divers.
[0214] The submersible 1012 can be a tracked vehicle that moves along the sea bed, or a
frame that is deposited on the sea bed, or a submersible vehicle like a submarine.
[0215] The submersible can also perform functions such as (i) laying cables on the sea bed,
for example for connecting the floating turbine assembly 104 to a power grid; (ii)
securing cables to the sea bed, for example by laying concrete mattresses or using
staples; (iii) making sub-sea electrical connections; (iv) inspecting various aspects
of the apparatus underwater, for example the various devices, anchors or cables; (v)
performing site surveys, for example performing geophysical or geotechnical surveys
such as sub-bottom profiling or taking core samples of the sea bed; (vi) removal of
debris, for example removing debris from a site prior to installation of anchors or
cables.
[0216] Through the use of a tether, the complexity of the submersible 1012 is reduced as
no power storage or generation needs to be located on-board the submersible 1012 and
communication between the submersible 1012 and the surface vessel 1011 is simplified.
Without a tether for communication, suitable underwater wireless communication needs
to be provided with the associated difficulty and expense in doing so.
[0217] In addition, the surface vessel 1011 can also be controlled remotely, for example
through a suitable wireless communications channel such as a mobile phone data connection
or satellite communications link, While the surface vessel 1011 can be controlled
remotely, a remote user is most concerned with controlling the submersible so the
present invention provides that the surface vessel 1011 is configured by default to
follow the submersible. As such, when the remote user moves the submersible 1012,
the surface vessel is configured to move itself so that the submersible stays within
communication range of the surface vessel. There are a variety of options for what
the communication range is considered to be. As the submersible 1012 is tethered to
the surface vessel, the maximum communication range is the maximum length of the tether.
It is more likely that the communication range will be set to be shorter than the
maximum length of the tether to prevent damage to the tether due to wear over time
or through excess tension on the tether. It is likely, therefore, that a set of threshold
values are to be used by the arrangement such that the threshold is set so that the
surface vessel 1011 does not need to move too frequently, i.e. a minimum threshold
for movement. It is also likely that a maximum threshold will be set to ensure that
the tension in the tether does not exceed a predetermined value where this predetermined
value is selected to reduce the likelihood of damage to the tether through excess
tension in the tether.
[0218] To determine the communication range, either the surface vessel 1011 or submersible
1012 can be provided with a range determiner to determine the distance of the submersible
1012 from the surface vessel 1011. This range determiner can be some form of tension
detection device configured to detect the tension in the tether 1013 such that, when
the tension in the tether 1013 is detected to reach a predetermined maximum threshold,
either the submersible 1012 has to stop moving away from the surface vessel 1011 or
the surface vessel 1011 has to move to shorten the distance between the surface vessel
1011 and the submersible 1012. By shortening this distance, the tension on the tether
1013 should be decreased within tolerances and sufficiently below the thresholds for
tension in the tether 1013.
[0219] Optionally, the surface vessel 1011 may be configured to orient itself, taking account
for example the conditions at the water surface and in the vicinity of the vessel
1011 and tether 1013, to reduce the drag and/or tension in the tether 1013. This has
the advantage of minimising the tension in the tether 1013 in normal operation, allowing
a greater degree of flexibility and range for the submersible as the tether 1013 will
suffer reduced tension from drag and/or tension caused by the surface vessel 1011.
[0220] Referring now to Figure 28, another aspect of the present invention will be described
in more detail. Figure 28 shows a simplified system diagram for the electronics in
an embodiment of the present invention. It shows one or more turbines 1050, which
generate alternating current (AC) from the water current(s). These provide one or
more lines of respective AC 1060 to one or more inverters 1051, which convert the
AC into direct current (DC). The respective one or more lines of DC 1061 are then
provided to an Active Front End 1052 that reconverts the DC into AC. The AC 1062 is
then provided to a power grid 1053 which is usually located on land and remotely from
the floating turbine assembly 104.
[0221] Figure 28 also shows some optional features of this system diagram. One or more battery/batteries
1056 are provided to power any onboard systems 1057 when the turbines are not generating
power. With a connection to a power grid 1053, however, the system may not need a
battery 1056 as power can be drawn from the power grid 1053 instead. It should be
noted that a charger/inverter 1054 is provided to convert the AC 1062 back to DC 1063
to charge the batteries 1056 or power the onboard systems 1057.
[0222] Another optional feature is a mast 1059 that enables external power to be provided
into the floating turbine assembly 104, for example to charge the on-board battery/batteries
1056 or to provide power to the hydraulic systems 1058 or on-board electronic systems
1057. It is expected that this external power would be a temporary connection, for
example provided by a surface vessel, where the batteries 1056 do not have sufficient
charge and a connection to a power grid 1053 is not available. The mast 1059 can connect
to the buoy 1004 via the buoy line 1005 and power can be provided to the buoy to power
a light. The buoy 1004 may further comprise a radar reflector. The mast 1059 can also
provide power to a wireless data link that can be provided on the buoy 1004. The buoy
line 1005 can also comprise a data cable to link the wireless data link on the buoy
to the floating turbine assembly 104. The buoy 1004 can also provide a means to control
the floating turbine assembly 104 in the case of system failure, including control
of the hydraulics systems 1058 and on-board electronic systems 1057. The buoy 1004
can also provide a means to provide air into the variable buoyancy tanks. The buoy
line 1005 is attached to a mast that provides clearance for the line 1005 from the
blades 132.
[0223] Finally, the hydraulic systems 1058 are connected to the AC 1062 to enable the floating
turbine assembly 104 to power the hydraulics required to operate the winches, rotate
the turbines 110 and operate the hydraulic rams that are used to make fine adjustments
to the mooring lines.
[0224] The apparatus is configured to energise a DC rail in the electronic systems shown
in Figure 28 to begin production of power from the turbines when sufficient water
flow is detected, by for example a water flow sensor. To energise the DC rail, power
can be drawn from either the battery 1056 or the connection to a power grid 1053.
[0225] Another aspect of the invention relates to an embodiment of the invention that is
provided with upstream turbines, i.e. turbines that only generate power when facing
upstream. In this embodiment, and where the floating turbine assembly 104 is sited
in a location that has bi-directional water flow, e.g. tidal flow, the present invention
can be configured to provide power generation in both directions. Referring to Figure
29, there is provided a floating turbine assembly 104 of the present invention tethered
using an anchoring system having four mooring lines or mooring line runs 145a, b,
c & d attached at four anchoring points 148a, b, c & d fixed to the sea bed. The floating
turbine assembly 104 also has a buoy 1004 connected to the central buoyancy device
107 with buoy line 1005. The current direction is in the direction of arrow Z and
so the turbines 110 and blades 132 are oriented upstream into the current Z in order
to generate power.
[0226] Referring now to Figure 30, the same floating turbine assembly 104 is shown generating
power from current Z. At some point, the current Z will slow down and stop, then the
current will change direction and instead increasingly flows in direction of arrow
Y.
[0227] The floating turbine assembly 104 is provided with a mechanism for detecting water
speed, for example a water speed sensor. As an alternative mechanism, the turbines
110 and blades 132 may themselves be used instead of a water speed sensor, for example
either through monitoring the rotations of the blades 132 or through monitoring the
level of power generation by the turbines 110.
[0228] Referring now to Figure 31, at some point the speed of current Z will be determined
to be too low to generate power, for example this can be predetermined and set by
a threshold value determined by the specific combination of equipment used on-board
the floating turbine assembly 104. When the speed of the current falls below the threshold
the turbines 110 and blades 132 are rotated into an upright orientation, i.e. facing
towards the surface of the water. An advantage of this orientation is a reduced need
for a braking mechanism for the blades. Another potential advantage is that the upright
orientation enables the blades to be braked without a braking mechanism. Some underwater
turbines are not provided with a braking mechanism, so this will prevent the blades
being rotated by a current as the blades will be facing out of the direction of a
steady current. Some turbines only have brakes that can prevent the blades 132 moving
(as opposed to slowing and stopping the blades once they have started moving) so this
arrangement will remove the motive force that rotates the blades such that the braking
mechanism can then lock the blades still, or prevent continuous movement that would
cause the turbines to start generating power.
[0229] In addition to this arrangement above, one or more inverters can be provided within
the floating turbine assembly 104. The inverter(s) are connected to the one or more
turbines 110 and can be set to temporarily draw an increased amount of power from
the turbines to provide electrical braking to slow the blades 132.
[0230] When sufficient current is detected in direction Y, the turbines 110 and rotors 132
are rotated as in Figure 32. The rotors 132 and turbines 110 in Figure 32 now face
into the current Y and can generate power once there is sufficient water speed.
[0231] As noted above, each feature may be provided independently and applied to other embodiments
or aspects.